Research has been performed on an optical device using an optical transmission line which optical device is suitable for optical communication systems and the like. It is expected that such an optical device will be applied to an optical data bus sheet for data exchange between optical circuits and applied to an optical splitter for splitting a signal beam and an optical combiner for combining signal beams. Of the optical transmission lines, a multi-mode optical transmission line, which is inexpensive compared to a single-mode optical transmission line, can take the place of a conventional electronic circuit.
An example of the multi-mode optical transmission line is a sheet-form multi-mode optical transmission line. For example, Document (3) discloses an optical bus circuit board provided with: a sheet-form transparent medium whose refractive index is homogeneous; a laser diode array that makes a signal beam incident on an incident end surface of the transparent medium; and a photodiode array that receives the signal beam having exited from the exit side end surface of the transparent medium. In the optical bus circuit board disclosed in Document (3), the incident beam emitted from the laser diode array is repetitively totally reflected in the direction of the thickness and in the direction of the width inside the transparent medium, exits from the entire area of the exit side end surface as the exiting beam, and is received by the photodiode array.
Moreover, Document (2) discloses, like Document (3), an optical splitter provided with: a sheet-form transparent medium whose refractive index is homogeneous; a laser diode that makes a signal beam incident on the transparent medium; and a plurality of optical fibers that receives the signal beam having exited from the transparent medium. In the optical splitter described in Document (2), a light diffusing layer is provided on the incident side end surface so that the signal beam is efficiently diffused inside the transparent medium within a short distance. In Document (2), the incident beam is also repetitively totally reflected in the direction of the thickness and in the direction of the width within the transparent medium, exits from the entire area of the exit side end surface as the exiting beam, and is received by the photodiode array.
Moreover, Document (1) discloses a sheet-form optical data bus having a refractive index distribution such that the highest refractive index is provided at the center in the direction of the thickness and the refractive index is decreased with distance from the center. In the optical data bus described in Document (1), the mode dispersion of multiple modes is reduced by the refractive index distribution. In Document (1), the incident beam also exits from the entire area of the exit side end surface as the exiting beam.
On the other hand, there is a technology in which an optical waveguide that transmits a signal beam in multiple modes in the direction of the width is disposed between a single-mode optical transmission line on the incident side and a single-mode optical transmission line on the exit side. This optical waveguide has a predetermined size L in the direction of the length which size L is determined by a uniform refractive index n of the optical waveguide, the basic mode width W0, in the direction of the width, of the optical waveguide and the wavelength λ of the transmitted signal beam. The optical waveguide generates the exiting beam by the eigenmodes of the signal beam interfering with each other in the direction of the length, based on the size L in the direction of the length (Documents (4) to (8), (11)).
Moreover, in recent years, in the field of optical communications, a wavelength division multiplexing (referred to also as WDM) method has been examined in which, in order to increase the communication capacity, a plurality of signals is superimposed on a signal beam of different wavelengths to be multiplexed and transmitted on the same optical transmission line. In the WDM method, optical devices such as an optical demultiplexer that demultiplexes signal beams of different wavelengths and an optical multiplexer that multiplexes signal beams of different wavelengths play an important role.
A conventional example is known that realizes such an optical demultiplexer and an optical splitter by use of the technology in which an optical waveguide that transmits a signal beam in multiple modes in the direction of the width is disposed between a single-mode optical transmission line on the incident side and a single-mode optical transmission line on the exit side (Documents (9), (12) to (15)). These conventional optical demultiplexer and optical splitter are connected to the incident side single-mode waveguide and to the exit side single-mode waveguide, and are provided with the optical waveguide that transmits a signal beam in multiple modes in the direction of the width. In the optical devices described in Documents (9) and (12) to (15), a multiplex signal beam of two wavelengths that are different from each other is transmitted in the incident side single-mode waveguide and made incident on the optical waveguide. The size, in the direction of the width, and the size, in the direction of the length, of the optical waveguide are set so that the exiting beam is generated in a different position on the exit end by the eigenmodes of the signal beam interfering with each other in the direction of the length.
Moreover, Document (10) discloses a method of manufacturing an optical device provided with an incident side beam converter, an optical waveguide and an exit side beam converter. The optical device manufacturing method of Document 10 describes that the optical waveguide is formed by enclosing a fluid material in a glass substrate. Moreover, Document (10) particularly discloses that the incident side beam converter and the exit side beam converter are provided with a refractive index distribution by successively laminating materials having different refractive indices (see Document (10), FIG. 4 and the corresponding description).
In a case where the multi-mode optical transmission line is made of a homogeneous medium, when a signal beam is transmitted, the physical optical path length (phase velocity) differs among the modes. For this reason, a phenomenon occurs in which the intensity distribution of the exiting beam varies depending on the length of the optical transmission line.
Moreover, when the length of the multi-mode optical transmission line is long to an extent that exceeds 100 mm, since the group velocity differs among the optical paths, a phenomenon occurs in which the signal waveform of the transmitted beam changes.
As described above, when a mode dispersion occurs which is a phenomenon in which the phase velocity or the group velocity differs among the modes, the signal beam cannot be transmitted while the intensity distribution of the incident beam is maintained to the exit side.
To solve the above-mentioned problem, an optical transmission line provided with a refractive index distribution is proposed. A signal beam propagating through a medium having a refractive index distribution draws a curved (meandering) beam locus based on the refractive index distribution. By applying this phenomenon, even if the physical optical path lengths of the optical paths are different from each other, the optical path length thereof can be made the same as each other by the difference in refractive index. Therefore, by appropriately setting the refractive index distribution, a multi-mode optical transmission line can be obtained since the mode dispersion is suppressed.
For example, Document (1) describes an optical device provided with laminated sheet-form optical transmission lines and having a refractive index distribution in the direction in which the sheet-form optical transmission lines are laminated. The sheet-form optical transmission lines described in Document (1) are capable of transmitting a gigabit-class high-frequency signal in multiple modes since the mode dispersion is suppressed by the refractive index distribution.
Such an optical device requires a structure for making a signal beam incident on the sheet-form optical transmission lines and making the signal beam to exit from the sheet-form optical transmission lines. In the optical device described in the above-described Document (1), a signal beam is made parallelly incident in the signal beam transmission direction from one end of the sheet-form optical transmission lines, and parallelly exits in the signal beam transmission direction from the other end of the sheet-form optical transmission lines (FIGS. 1 and 9 of Document (1)).
Moreover, a technology is known in which the optical waveguide (sheet-form optical transmission line) is provided with a mirror for perpendicularly bending the optical axis of the signal beam and the optical waveguide is coupled to the outside (FIGS. 1 and 2 of Document (16)). In the optical waveguide described in Document (16), the signal beam incident from a direction perpendicular to the transmission direction is bent by a mirror disposed at 45 degrees from the signal beam transmission direction and is incident on the optical waveguide. Moreover, the signal beam transmitted through the optical waveguide is bent by a mirror disposed at 45 degrees from the signal beam transmission direction and exits in a direction perpendicular to the transmission direction (see FIGS. 1 and 2 of Document (16)).
List of the Documents
(1) Japanese Laid-Open Patent Publication No. 2000-111738(FIG. 3)
(2) Japanese Laid-Open Patent Publication No. 2000-329962(FIG. 2)
(3) Japanese Laid-Open Patent Publication No. 2001-147351(FIG. 1)
(4) Japanese Laid-Open Patent Publication No. 2003-050330(FIG. 1)
(5) Japanese Laid-Open Patent Publication No. 2001-183710(FIG. 1)
(6) Japanese Laid-Open Patent Publication No. Hei 1-156703(FIG. 1)
(7) U.S. Pat. No. 4,087,159 (FIG. 1)
(8) U.S. Pat. No. 4,950,045 (FIG. 1)
(9) Japanese Laid-Open Patent Publication No. Hei 8-201648 (pages 2 to 5, FIG. 11)
(10) Japanese Laid-Open Patent Publication No.2003-043285 (FIG. 4)
(11) Lucas B. Soldano and Eric C. M. Pennings, “Optical Multi-Mode Interference Device Based on Self-Imaging: Principles and Applications”, Vol. 13, No.4 Journal of Lightwave Technology, April, 1995
(12) F. Rottmann, A. Neyer, W. Mevenkamp, and E. Voges, “Integrated-Optic Wavelength Multiplexers on Lithium Niobate based on Two-Mode Interference”, Journal of Lightwave Technology” Vol. 6, No. 6 June, 1988
(13) M. R. Paiam, C. F. Janz, R. I. MacDonald and J. N. Broughton, “Compact Planar 980/1550-nm Wavelength Multi/Demultiplexer Based on Multimode Interference” IEEE Photonics Technology Letters, Vol. 7, No. 10, October, 1995
(14) K. C. Lin and W. Y. Lee, “Guided-wave 1.3/1.55 μm wavelength division multiplexer based on multimode interference”, IEEE Electronics Letters, Vol. 32, No. 14, Jul. 4, 1996.
(15) Baojun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Jie Qin and Xun Wang, “Low-Loss 1×2 Multimode Interference Wavelength Demultiplexer in Silicon-Germanium Alloy” IEEE Photonics Technology Letters, Vol. 11, No. 5, May, 1999
(16) Japanese Laid-Open Patent Publication No. Sho 62-35304(FIG. 1, FIG. 2)